Basecoat-clearcoat coatings, or color-plus-clear composite coatings, are widely used, especially in the automotive industry, because of their exceptional appearance. Automotive coatings must not only have the desirable appearance properties of high gloss, depth of color, distinctiveness of image, and so on; but they must also be resistant to marring, scratching, etching, or spotting from environmental depositions, chalking, and other forms of film degradation. It is particularly critical in a color-plus-clear composite coating to have a clear coat layer that is resistant to film degradation.
In addition to the considerations of appearance and durability, a coating must be formulated to minimize the amount of regulated volatile organic compound emissions from the painting process. For a thermoset solventborne system this is usually accomplished by employing low molecular weight resins and crosslinking agents. One drawback to this approach is that such coatings systems have poorer rheological properties during application and the curing bake. When the coatings are heated in order to cause a reaction between the resins and crosslinking agents, the viscosity of the low molecular weight resins is reduced. These resins tend to flow on the coated substrate causing sagging, slumping, so-called fat edges, and other appearance problems before the coating has achieved a sufficient level of cure to prevent further movement.
Difficulties with controlling rheology have been a problem particularly with coatings compositions that are cured using blocked polyisocyanates as crosslinkers. Although coatings compositions using polyisocyanate crosslinkers are highly preferred for the excellent appearance and durability of the films they produce, such systems must usually be heated to temperatures of 290.degree. F. (143.degree. C.) and higher in order to de-block the polyisocyanate and thereby initiate cure. At these temperatures, the tendency of the low molecular weight principal resins to flow is increased. Thus, the coating composition flows more than is desirable before there is sufficient crosslinking to set up the coating film. The excessive flow leads to the appearance problems in the cured coating mentioned above.
In U.S. Pat. No. 5,342,888, Sudo describes the cure of hydroxyl-functional acrylic resins with a combination of blocked aliphatic polyisocyanates and alkyl-etherified melamine resins. According to Sudo, coatings from acrylic resins cured only with the alkyl-etherified melamine resin have poor resistance to acid rain deterioration, while such coatings cured only blocked polyisocyanates have good acid resistance but poor low-temperature cure and workability. Sudo teaches that the proportion of melamine resin to the blocked polyisocyanate must be from 50/50 to 95/5 by weight to avoid, on the one hand, inadequate cure and, on the other hand, poor acid resistance However, because the coating compositions of Sudo form coatings that still have a substantial amount of ether linkages from the aminoplast-hydroxy cure, such coatings will show significant deterioration from acid rain etching.
U.S. Pat. No. 4,102,847, Hartmann et al., and U.S. Pat. No. 4,892,906, Pham et al., disclose coating compositions similar to that of Sudo. Both the Hartmann and the Pham references teach hydroxy-functional resins cured with mixtures of melamine and isocyanate. Again, the coating compositions of these references form coatings with a substantial amount of ether linkages and thus will show significant deterioration from acid rain etching.
Carbamate-functional reactive diluents are described in WO 87/00851, Hoy et al.; U.S. Pat. No. 5,115,015, Richey, Jr., et al.; U.S. Pat. No. 4,814,382, Hoy et al.; U.S. Pat. No. 4,677,168, Hoy et al.; and U.S. Pat. No. 4,520,167, Blank et al. The carbamate-functional reactive diluents are low molecular weight compounds with one carbamate group. These materials are mono-functional in the systems disclosed and therefore cannot build up viscosity fast enough at the beginning of the cure cycle to prevent the sagging and other flow problems mentioned above.